1. Field of the Invention
The present invention relates to a toner for use in an electrophotographic image forming apparatus such as copying machines, electrostatic printing, facsimiles, printers and electrostatic recording, a developer using the toner, and an image forming method using the toner.
2. Description of the Related Art
Conventionally, in electrophotographic image forming apparatuses and electrostatic recording apparatuses, electric or magnetic latent images are developed into images by the use of toner. For example, in an electrophotographic process, an electrostatic image or latent image is formed on a photoconductor, and then the latent image is developed by use of toner to form a toner image. Typically, the toner image is transferred onto a recording medium such as paper and then fixed thereto by means of heating or the like.
In an image forming apparatus employing the heat fixing system, a large amount of electric power is necessary in the course of heat melting and fixing a toner onto a recording medium such as paper. Thus, from the viewpoint of energy saving, the low temperature fixing ability of the toner is one of the important properties.
To achieve fixation of a toner at low temperature, it is important to control the heat properties of a binder resin, which accounts for a large percent of a toner. For example, by adding a material which is compatible with the binder resin and exhibits plasticization effect (hereinafter, referred to as a fixing aid) to a toner, the glass transition temperature (Tg) of the binder resin can be decreased. There has been proposals of toners, for example, a toner containing a crystalline resin, a certain polycarbonate resin, a polyarylate resin, and a polyvinyl acetal resin (see, Japanese Patent Application Laid-Open (JP-A) No. 2006-267980), and a toner containing a cycloolefin copolymer resin and a crystalline resin (see, Japanese Patent Application Laid-Open (JP-A) No. 2006-276074). However, when the glass transition temperature of the binder resin is excessively decreased, such problems occur that the heat resistant storage stability is degraded, and the toner is solidified in an image forming apparatus. Generally, there is a trade-off relation between the low temperature fixing ability and the heat resistant storage stability of a toner, and satisfaction of both properties is an important task in the development of a toner.
The aforementioned problems have been studied, and many proposals have been made. For example, JP-A No. 2007-199314 proposes a core shell-type toner, in which a surface of a core particle containing a first binder resin, a colorant and a plasticizer is coated with shell particles each formed of a second binder resin. This toner is designed to satisfy both the low temperature fixing ability and a mechanical strength, by use of the plasticizer and formation of the core shell structure. However, as the binder resin is compatible with the plasticizer in the toner from the time when toner particles are formed, the binder resin is plasticized, and the heat resistant storage stability is degraded.
Moreover, JP-A No. 08-328302 proposes a toner containing a matrix phase formed of a resin containing a styrene monomer as a structural unit and a domain phase formed of a resin containing an acrylic acid monomer as a structural unit dispersed in the matrix phase, wherein at least any one of the phases contains a fixing aid, and the relation between the resin constituting two phases, with respect to the molecular weight and the glass transition temperature are defined. This toner is designed to satisfy both the low temperature fixing ability and the heat resistant storage stability, by controlling affinity between the fixing aid and the binder resin to selectively affect softening of the resin by the use of the fixing aid on one phase. However, a resin, which forms a phase containing no fixing aid, is not softened, and there is such problem that the low temperature fixing ability cannot be sufficiently achieved.
JP-A No. 2008-281884 proposes a toner, which contains, as a binder resin, a polyester resin obtained by condensation polymerization between an alcohol component containing 1,2-propane diol and a carboxylic acid component containing a refined rosin, and a fixing aid existing as a crystalline domain. JP-A No. 2007-233169 proposes definition of the relation between a first temperature increase value and a second temperature increase value in the endotherm of a melting peak derived from a fixing aid in DSC measurement of a toner. Japanese Patent (JP-B) No. 4365347 proposes a toner containing a plasticizer which is incompatible with a binder resin and a wax, wherein the a plasticizer and the wax are incompatible with each other, and the plasticizer becomes compatible with the binder resin by heating at a glass transition temperature of the plasticizer or higher, or at a glass transition temperature of the resin or higher. In JP-A Nos. 2008-281884 and 2007-233169, and JP-B No. 4365347, both the low temperature fixing ability and the heat resistant storage stability are satisfied at a certain level. However, the low temperature fixing ability is demanded for further improvement, from the viewpoint of energy saving.
A toner contains a binder resin in an amount of 70% or more. Since most of the conventional binder resins are made from oil resources, there are concerns of depletion of the oil resources and the issue of global warming caused by discharge of a carbon dioxide gas into the air due to heavy consumption of the oil resources. If a binder resin can be synthesized from a plant (so-called, a plant-derived resin) which grows by utilizing carbon dioxide gas in the air, the carbon dioxide gas can be circulated. Namely, there is a possibility of preventing the global warming and the depletion of the oil resources. Various toners using plant-derived resins as binder resins have been proposed.
As widely-used, and easily obtainable plant-derived resins, polylactic acid is exemplified. However, polylactic acid is very hard resin and difficult to use for toner production by a pulverization method. Moreover, since poly-L-lactic acid or poly-D-lactic acid alone has high crystallinity, it has extremely low solubility in an organic solvent, and is difficult to use for toner production by a polymerization method such as a suspension method by dissolving resins. On the other hand, JP-A No. 2008-262179 discloses that poly-L-lactic acid and poly-D-lactic acid are mixed together so as to decrease crystallinity and improve their solubility in an organic solvent.
However, polylactic acid contains low molecular weight of monomer unit, and a large number of polar groups per unit molecule, so that in the case where a toner is produced using polylactic acid(s) with reduced crystallinity, the toner is affected by moisture to a greater extent than in the case of polylactic acid(s) with high crystallinity. This leads to degradation of the storage stability of the toner, variation in the fluidity of the toner caused by moisture absorption and difficulty in controlling the charge amount. It is particularly difficult to maintain a charge amount at a certain level under a condition which can belong to anywhere between a low-temperature, low-humidity condition and a high-temperature, high-humidity condition, and thus there are problems of unstable image density.
Accordingly, a toner which has a wide range of fixing temperature, is superior in low temperature fixing ability, heat resistant storage stability, haze degree, has high stability of image density against change in usage environment, such as temperature, humidity, etc., and contains a polylactic acid, and the related techniques have not yet been achieved, and further improvements and developments are still desired.